In general, an air conditioner for vehicles is a car trim, which is installed in the vehicle in order to secure a driver's front and rear visual field by heating and cooling the inside of the vehicle in the summer season or the winter season or removing frost from a windshield in case of rain or the winter season. Since such air conditioner includes a heating device and a cooling device together, the air conditioner heats, cools or ventilates the inside of the vehicle through the steps of selectively inducing the indoor air or the outdoor air through a blast unit, heating or cooling the air, and blowing into the vehicle.
Such an air conditioner is classified into a three-piece type where a blast unit, an evaporator unit, and a heater core unit are disposed independently, a semi-center type where the evaporator unit and the heater core unit are embedded in an air conditioning case and the blast unit is mounted separately, and a center-mounting type where the three units are all embedded in the air conditioning case.
Furthermore, an air conditioner having a two-layer air flow structure has been developed to secure a defogging efficiency during heating and to maintain a heating efficiency of high quality. That is, the conventional air conditioner for the vehicles introduces cold outdoor air of low humidity thereto since the cold outdoor air is effective to remove frost stained on a window of the vehicle during heating and traveling in the winter season, but it causes lowering of the room temperature. In the above case, the air conditioner having the two-layer air flow structure realizes a two-layer air flow of indoor air and outdoor air that the outdoor air is supplied to the upper part of the vehicle and the indoor air is circulated in the lower part of the vehicle, whereby the air conditioner can effectively remove frost using fresh outdoor air of low humidity supplied to the upper part, provide fresh outdoor air to passengers and a driver, and maintain the heating efficiency of high quality by supplying warm indoor air to the lower part of the vehicle.
Referring to FIG. 1, the air conditioner having the two-layer air flow structure will be described in brief as follows.
As shown in the drawing, the air conditioner 1 having the two-layer air flow structure includes: an air conditioning case 10 having an air passageway 10a of a predetermined form formed therein by a partition wall 10b, an indoor air inlet 14a and an outdoor air inlet 14b formed at an entrance side of the air passageway 10a and divided from each other by a partition wall 14c, and a number of air vents 15 formed on an exit side of the air passageway 10a. Here, the air vents include a defrost vent 16, a face vent 17, a front floor vent 18, a rear floor vent 19, and so on.
A blower unit 5 is mounted on an entrance side of the air conditioning case 10, and includes: a first blast fan 6 for sending outdoor air to the outdoor air inlet 14b; and a second blast fan 7 for sending indoor air to the indoor air inlet 14a. 
Inside the air conditioning case 10, an evaporator 2 and a heater core 3 are respectively mounted at a predetermined interval from the indoor and outdoor air inlets 14a and 14b. In front of the heater core 3, first and second temperature control doors 11 and 12 are mounted for selectively opening and closing a front air passageway 10c of the heater core 3 and an upper air passageway 10d. 
Here, the second temperature control door 12 has the rear end portion rotatably mounted at a central height of the heater core 3 so as to open and close the heater core 3 from the center to the lower portion. In this instance, a free end portion of the second temperature control door 12 rotates to a central portion of the evaporator 2 corresponding to the partition wall 14c to divide the outdoor air and the indoor air from each other.
In addition, the first temperature control door 11 has the rear end portion rotatably mounted at the upper end portion of the heater core 3 and a free end portion overlapped with, a partition wall of the upper air passageway 10d or the second temperature control door 12 to selectively open and close the heater core 3 from the center to the upper portion and the upper air passageway 10d. Moreover, the first and second temperature control doors 11 and 12 are rotatably operated in interlock with each other by an actuator (not shown).
Meanwhile, a number of mode doors 13 are mounted on the exit side of the air passageway 10a to control air flowing toward the air vents 15.
Therefore, in a two-layer air flow mode, as shown in FIG. 1, the first temperature control door 11 closes the upstream side air passageway 10d of the heater core 3 and the free end portion of the second temperature control door 12 rotates to the central portion of the evaporator 2, so that the indoor air and the outdoor air introduced in their separated state through the indoor air and outdoor air inlets 14a and 14b flow to the evaporator 2 and the heater core 3 in the separated state, and then, the outdoor air flows toward the defrost vent 16 or the face vent 17 and the indoor air flows toward the front floor vent 18 or the real floor vent 19, whereby the air conditioner can secure a defogging efficiency and keep an improved heating performance.
However, the conventional air conditioner 1 adjusts temperature by using the two temperature control doors 11 and 12 and has to simultaneously operate the two temperature control doors 11 and 12 in interlock with each other to realize the two-layer air flow structure. So, the conventional air conditioner 1 has a problem in that its structure is complicated and manufacturing costs are increased due to the complicated components.